A huge differences between writing a novel and writing a screenplay is the ability to get inside the head of a character. In a novel you can do this easily, whether writing in the first person or third person.
In a screenplay you can't do this, except with voice-over. And voice-over is a tricky beast that's often misused.
Not being able to hear the thoughts of a characters is one of the biggest obstacles a writer faces when working on a screenplay. The screenwriter should only put on the page what the audience will see or hear. Period. Any inner thoughts have to be spoken as dialogue or they will not be communicated to the audience.
The movie is based on the 2001 novel by Michel Farber, and this brings us to that age old question: what is better, the book or the movie? And what if the answer was: both. Both of them are great, only in vastly different ways.
The novel is written in the third person point of view, but might as well have been in the first person as it is told through the eyes of the completely through the eyes of the protagonist, an alien female named Isserly, horribly surgically altered and sent to Earth to seek out male human victims for a horrific purposes.
The aliens in the novel are embodied with the feelings and motivations of human beings. In fact, in the novel, the aliens have appropriated the term "human being" for themselves and refer to Earthlings as "vodsels," beings considered little different from dogs or sheep.
Like all protagonists in a good piece of fiction, the novel takes Isserly on a journey of change from the creature we meet at the opening to the one at the haunting, disturbing end. The journey in the novel is fairly complex, with twists and turns and plenty of interaction of secondary characters as well as gobs of often philosophical discussion between Isserly and her co-conspirators.
But it is Isserly's internal monologue that is the spine of the novel, and the vehicle by which we learn everything about her. Her embarrassment and lingering horror at having been surgically altered to appear human, the pain these modifications and the thick glasses she must wear to disguise her over-sized eyes cause, her refusal to let herself admit that vodsels are intelligent creatures deserving of respect.
So they went the opposite direction, took a pass on voice over and pretty much decided they didn't need any dialogue in the picture, either. (Sure, there is dialogue, but none of it is exposition, that is dialogue that explains what in holy fuck is going on.) The story is told visually in a slow, hypnotic, and unrelentingly precise dance between camera, lighting and acting. The alien characters in the film, Johansson and a guy who wears a motorcycle suit with a suspicious hump everywhere he goes, never exchange a word.
Johansson gives the performance of a lifetime, conveying the most dramatic narrative developments with nothing more than facial expression.
In the novel, Isserly is an alien, but an alien working with human motivations. In the film, Johansson's unnamed alien is just that. Utterly, unrelentingly alien. While in her banter with men on the streets of Glasgow she conveys a warm sensuality (and apparently many of the men in the film are random strangers Johansson pulled up next to, then attempted to get into her van, all recorded on hidden camera.) Once Johansson's alien is done with her human performance she turns into a cold, ruthless, utterly inhuman creature. There is one scene, on a beach, that is so unrelentingly cruel as to be almost beyond description.
Yet that cold facade breaks, slowly, and with the most unexpected and disturbing of consequences.
What's wonderful about both versions of this story is how each creator seized upon the most powerful element of their art form and pushed it to the maximum. In a novel, that is the ability to live inside the head of another sentient being, thinking their thoughts, feeling their feelings, living their emotions. In the film, Glazer seizes on the true strength of film, telling a story through images.
But in the end, despite their differences, both of these fine works bring us to the same place: forcing the reader/viewer to confront what it truly means to be human.